This invention concerns a method for the electromagnetic stirring of the liquid metal in electric arc furnaces and the relative stirring device.
The invention is applied in the siderurgical field and in particular in direct current electric arc furnaces, in order to obtain a controlled electromagnetic stirring of the liquid metal, both during the melting stage and the refining stage. This stirring is intended to facilitate the optimum performance of the various steps in the process, to improve the quality of the final product and to save energy.
The invention makes it possible to operate both when the electric arc is switched on, during the melting process, and also when the electric arc is switched off, at the end of the melting process, in order to maintain the liquid metal at temperature during the refining step.
The invention is also used, as a complementary function to the electromagnetic stirring, to generate a controlled rotation of the electric arc in order to distribute and regulate its action in a uniform manner over all the periphery of the furnace, thus preventing, in particular, the formation of hot points which cause a deterioration and premature wear of the refractory material inside the furnace.
The state of the art covers the need to supply devices to mix and stir the liquid metal in electric arc furnaces in order to render the bath homogenous and to accelerate the chemical reactions of the melting process so as to obtain high quality steels with reduced working cycles.
Direct current electric furnaces include a central cathode which cooperates with a plurality of anodes suitably distributed on the floor and causes the generation of the electric arc which melts the raw material loaded into the furnace.
The electric current which is fed to generate this electric arc induces a magnetic field in the molten metal; the magnetic field interacts with the current itself and generates electromagnetic body forces which tend to partially stir the molten metal in a direction of preferential rotation.
Although in direct current electric arc furnaces the entity of the electric current and of the magnetic field induced are high, the electromagnetic forces produced are only able to achieve a modest mixing of the molten metal, which is not sufficient to satisfy the operational needs of the bath, in terms of homogeneity and uniformity, such as are required to optimise the melting process.
There are pneumatic stirring devices known to the state of the art which, although they provide an energetic stirring of the liquid metal, only do so locally; this stirring action is not therefore sufficient to homogenise the physical and chemical conditions of the bath in relatively short times.
Moreover, because of the type of gas used, these devices are either too expensive, (for example, when the gas argon is used), or else they risk compromising the final quality in certain types of product (for example when azote is used).
On the other hand however, there are no efficient mechanical means available to stir the liquid metal, both because of the high temperatures in the furnace and also because of the heterogeneous raw materials--which would compromise the functioning of the mechanical means and cause them to frequently break or malfunction.
GB-A-1.067.386 describes a magnetic stirring system achieved by means of a magnet or solenoids fed with direct current and placed below or inside the furnace.
This system is based on the generation of electromagnetic forces acting on the bath of molten metal inside the furnace by means of the interaction between a radial magnetic field and a current with a substantially vertical direction.
The system proposed by GB'386 has the advantage that it obtains a more efficient stirring of the bath without having to use magnets or high powered devices.
However, the system is only able to provide a limited stirring capacity of the liquid metal and, to be more precise, it is not possible to control the speed of the liquid metal, and vary it according to the specific step of the melting cycle and/or according to its position with respect to the surrounding refractory walls, in order to prevent progressive and premature wear and erosion.
Moreover, the system proposed by GB'386 necessarily includes the electric arc and therefore it has no function whatsoever during those steps, for example, during refining, when the furnace is working with the arc switched off.
U.S. Pat. No. 4,149,024 describes a system to control the deviation and rotation of the arc which uses a rotating magnetic field generated by electromagnets fed with three-phase currents and arranged outside the furnace.
The electromagnets produce a migrating magnetic field which interacts with the current of the arc and generates such a force that it causes the deviation and rotation of the arc itself.
The system proposed by US'024 is not able to obtain the stirring of the bath of molten metal and therefore it does not achieve the main function sought by this invention.
DE-A-3.232.551 describes a particular furnace-ladle with three upper electrodes which can act both as cathodes and as anodes; the circuit is closed by means of the slag deposited above the bath.
DE'551 includes elements identified as coils located outside the refractory material and indicated simply as electromagnetic stirrers, but the document describes neither their structure nor their functioning.
The principal purpose of the patent is to improve the treatment and refining of metallic alloys by means of heating generated by the electric arc and using the electrolytic slag as a bottom electrode.
The document therefore has no pertinence whatsoever to the purposes of this invention as regards an improvement in controlling the electromagnetic stirring of molten metal and the position of the electric arc in the melting and refining steps of the furnace.
FR-A-2.331.233 describes a direct current furnace equipped with ferromagnetic cores fed with direct current so as to obtain a controlled rotation of the electric arc.
This document also includes sensors to monitor the wear on the refractory material and the temperature, which condition the controlled rotation of the electric arc with respect to the electrode.
The arc is made to rotate by regulating the frequency and the type of connection (single-phase, two-phase, three-phase) of the ferromagnetic cores in such a way as to generate a magnetic field which, by interacting with the current of the arc, generates a force which is able to make it rotate.
When there is an asymmetric charge, the device described in FR'233 allows the speed of the arc to be varied so that it is higher in the more exposed areas and lower in the other areas.
No mention is made of any possibility of an electromagnetic stirring of the molten metal.
The article "Neue Entwicklungen fur . . . " taken from Stahl und Eisen vol. 114, n.degree.. 8, Aug. 15, 1994, pages 75-77, describes a system which controls the process of an alternating current furnace.
The article describes a control system for the arc which controls and limits the deviation thereof; this is achieved by means of an optimum arrangement of the current-bearing cables around the furnace.
In order to obtain the desired electromagnetic stirring, an element referred to simply as a coil is used, placed below the hearth of the furnace, but it is not specified if this coil is AC or DC; the coil produces a magnetic field which, by interacting with the current of the arc, generates an electromagnetic force which is able to make the liquid metal rotate.
This system does not provide to obtain the rotation of the arc, and, in order to control the deviation of the arc, it does not provide for the use of controlled and interacting electromagnetic forces.
The system used for stirring substantially has the same disadvantages already mentioned with regards to GB-A-1.067.386.